Transformer for band-pass selectors



1 q 1939- M. CAWEIN TRANSFORMER FOR BAND-PASS SELECTORS Filed April 14, 1938 FIG.3.

FIG.2.

INVENTOR L7MjDlSON CAWEI B ATTORNEY Patented Sept. 26 1939 UNITED STATES PATENT OFFICE TRAN SFORIVIER FOR BAND-PASS SELECTORS aware Application April 14, 1938, Serial No. 201,923

9 Claims.

This invention relates to transformers for band-pass selectors and particularly to transformers for band-pass selectors adapted to pass a wide band of frequencies and having a substantially uniform response for all frequencies within the band, such as those selectors utilized in the signal-frequency or intermediate-frequency channels of television receiving apparatus.

10 In amplifiers for television signal-translating W apparatuai is a practical necessity to design 7 the interstage coupling circuits so that the response is substantially uniform over a wide band of frequencies, the width of the band depending upon the information to be transmitted in the picture. It has been found that, if uniform response is procured from stage to stage in such translating apparatus, there is no observable effect of phase distortion in the transmitted picture. On the other hand, if there occur large departures from a uniform response over the frequency band transmitted which are uncompensated in the individual stages of the apparatus, considerable distortion effects may appear in the reproduced picture. Therefore, arrangements for compensating for these departures in any manner other than in the individual stages are generally not entirely satisfactory. It is, therefore, preferable to minimize departures from a uniform response over the band of frequencies transmitted in every stage of the apparatus.

One of the simplest and most satisfactory networks for passing a wide band of frequencies consists of a double-tuned transformer in which the power factor of, and inductive coupling between, the transformer windings are relatively proportioned to the relative band width, the factors of proportionality depending solely upon the departure from uniform response over the band which may be tolerated. The term relative band width as used herein is defined as the ratio of the width of the band of frequencies passed by the selector within this departure to the mean frequency of the pass band. However, for a transformer design involving a given band width and a given mean frequency of the pass band and for a predetermined maximum departure from uniform response over the pass 5 band, the value of magnetic coupling and the power factor are very critical. It has heretofore been the practice, therefore, in designing such a transformer, to provide an arrangement for adjustably fixing the inductive coupling be- 55 tween the primary and secondary windings to aid in procuring the critical relation between circuit parameters. Such adjustments are, however, expensive if the transformer is to be constructed in the large quantities involved in commercial production. 5

It is an object of this invention to provide a transformer having completely predetermined characteristics for embodiment in a band-pass selector circuit having a predetermined relative band width and a predetermined departure from 10 uniform response over the pass band.

It is a further object of the invention to provide a transformer having relatively fixed primary and secondary windings for embodiment in a band-pass selector for passing a wide range 15 of frequencies with substantially uniform response for all frequencies within the band, the transformer having dimensions which can be readily predetermined from the known requirements of the selector. 20

In accordance with the invention, there is provided a transformer of the above-mentioned type having dimensions determined in accordance with formulae hereinafter given. The windings may effectively comprise a continuous 25 single-layer winding of a predetermined pitch and wound on a suitable cylindrical insulation form of predetermined diameter. The winding, after being wound on the form, can be broken at its mid-point to provide separate primary and 30 secondary windings for the transformer of a band-pass selector circuit having known characteristics and having a response which is relatively uniform or having a predetermined departure from a uniform response over a pre- 35 determined band of frequencies. Suitable terminals for the transformer can be provided in any conventional manner. Preferably, however, terminals for one of the windings are affixed to one end of the Winding form, while similar terminals 40 are provided on the opposite end of the winding form for the other transformer winding. The adjacent ends of the windings, that is, those provided by breaking the winding at its mid-point, are preferably brought out through the interior 5 of the winding form to their respective termina-ls.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing, Fig. 1 is a circuit diagram of a stage of amplification comprising the transformer of the invention; Fig. 2 is a side elevation of the transformer of Fig. 1 with a quarter section thereof removed; and Fig. 3 is an end elevation of the transformer of Fig. 2.

Referring now more particularly to Fig. 1, there is shown a stage of amplification for passing a wide band of frequencies with a relatively uniform response, comprising a vacuum tube 51 coupled to a second vacuum tube It by means of a double-tuned transformer II constructed in accordance with formulae hereinafter given. The transformer ll comprises a primary winding i 2 tuned by its inherent capacitance and that of the output circuit of tube 9, represented by condenser l3 shown in dotted lines. The transformer ll also includes a secondary winding I4 tuned by condenser l5, also shown in dotted lines. Condenser l5 may be a physical condenser or it may comprise the inherent capacitance of winding I l and input capacitance of vacuum tube It. Resistors l5 and I! are connected, respectively, in shunt with primary winding l 2 and secondary winding l4. Suitable biasing and operating potentials for the electrodes of vacuum tubes 9 and Ill may be provided in a conventional manner.

In considering the operation of the stage of amplification of Fig. 1, it will be seen that, if the circuit constants are suitably proportioned, a wide band of frequencies can be substantially uniformly amplified by the stage of amplification shown. The amount of coupling required between windings l2 and I4 and the power factor of each of the tunnel circuits l2, l3 and M, 55 for maintaining the response uniform, within a given departure, over a given relative bandwidth are, however, very critical. t has been found that a suitable transformer of predetermined characteristics can be constructed in accordance with a definite procedure and definite formulae.

The coefficient of coupling l0 and the power factor P between the primary and secondary circuits required in each of the primary and secondary circuits of a transformer to be utilized in the circuit of Fig. 1, in which the response of the selector circuit is maintained uniform, within a given departure, over the band of frequencies transmitted, are given substantially by the formulae:

2 P: W 1M (2) 2M /1 2 where:

W:width of the band of frequencies transmitted, in kilocycles.

M=mean frequency of the band of frequencies transmitted, in kilocycles,

P :power factor of primary and secondary circuits at the mean frequency M,

D :ratio of minimum-to-inaximum gain over the pass band (coefficient of departure).

The coefficient of coupling required in such a transformer to be utilized in television signaltranslating apparatus usually is between the limits of 0.5 and 0.1.

The transformer of this invention is constructed as shown in Figs. 2 and 3. The transformer comprises windings l2 and M wound in the same direction on a suitable cylindrical winding form it as a continuous single-layer winding which is subsequently broken at its midpoint. Windings I2 and I4 each has the same number of turns and the pitch of the winding is in accordance with the formulae hereinafter given, The wire size of windings I2 and I4 is not critical, providing the wire size is small with respect to the diameter of the winding, A suitable terminal 28 is provided for one end of winding l2 and the other end of the winding is brought out through the aperture 2| in winding form I9 to terminal 22. Similarly, winding I4 is provided with a terminal 23 for one end thereof while the opposite end, being that adjacent winding I2, is brought out through aperture 24 to terminal 25. The transformer is constructed and dimensioned substantially in accordance with the formulae:

d: t (1 1 (3) i/2kL;(1-ld) (4) where t =number of turns per inch of each of windings I2 and I4.

L :self-inductance in microhenries of each of windings I2 and I4.

d=outside diameter of winding form I9, in

inches.

N :number of turns of each of windings I2 and I4.

Equations 1 and 2 may be derived from fundamental relationships between parameters involved in coupled circuit theory. By appropriate transformations of expressions for valley-to-peak response and for relative-peak-separation (see: R, T. Beatty, Wireless Engineer, October 1932, pages 546-557; C. B. Aiken, Proceedings I.R.E., February 1937, page 230) it can be shown that:

Valley-to-peak response,

2PI +p and Relative-peak-separation,

approximately equal to for Values of The simultaneous solution of these two equations for It and P yields Equations 1 and 2 of this specification.

By means of simple inductance formulae (see Harold A. Wheeler, Proceedings I.R.E October 1928, page 1398), it is possible to derive expressions for the inductance of each half of a singlelayer solenoid and for the inductance of the entire coil, considered as coupled halves. The simultaneous solution of these two last mentioned expressions yields Equations 3 and 4 above.

It will be understood that, in proportioning a system in accordance with the above formulae, resistors I6 and I! are proportioned to obtain the power factor required for the primary and secondary circuits of the transformer. The total equivalent parallel resistance, R, across each of windings I2 and I4, including the resistances of R: 21rMCP where:

C=tuning capacitance in farads across each of the transformer windings.

It will be seen that the above formulae contain all the information which is required to determine the dimensions of a transformer to be used in the band-pass selector circuit of Fig. 1 for passing frequencies of a given relative band width and with a predetermined departure from a uniform response over the pass band.

In summary, the procedure in designing a transformer in accordance with the invention is as follows: the output capacitance (the capacitance of condenser l3) of the circuit to which the selector is to be coupled is determined. The means resonant frequency M of the selector system and the width W of the frequency band to be passed are, of course, known. The variation in the response of the selector from a constant value which can be tolerated is determined thus fixing the ratio D. The self-inductance of each of the transformer windings is such as to resonate at the mean frequency of the band with the capacitances of condensers I3 and 15, respectively, which may be made equal by padding adjustments. The coefficient of coupling required between windings l2 and I4 and the power factor of each of the resonant terminal circuits l2, I3 and I4, I5 are determined from Equations 1 and 2 above. Having determined the self-inductance of each of windings l2 and I4 and the coefficient of coupling 70 required, the dimensions of the transformer are determined in accordance with the Formulae 3 and 4.

As an example of a specific application of the invention, circuit constants and design dimen sions for a transformer for an intermediatefrequency selector of a television receiving apparatus may be computed as follows:

Mean frequency of band megacycles 7.8 Width of band of transmitted frequencies megacycles 2.5 Vacuum tube 9-type A306A-having a distributed output circuit capacitance of It is assumed that the input circuit of tube It] also has a distributed capacitance, as represented by condenser [5 of Fig. 1, of 13 t.

For a ratio of minimum-to-maximum gain of 0.945 over the band transmitted by the selector circuit, the coefficient of coupling required, in accordance with Equation 1 above, is found to be 70:0.32. v

The self-inductance L of each of primary and secondary windings I 2 and M, respectively, to resonate with a capacitance of 13 [L/Lf at 7.8 megacycles is 32 microhenries.

The wire for windings l2 and I4 is arbitrarily chosen as No. 38 B. 8r S. double silk covered.

The number of turns per inch t, for No. 38 double silk covered wire close wound, is 120.

From Equation 3 the diameter of the winding form to give the required coefficient of coupling is d=0.71 inch.

From Equation 4 the number of turns per winding to give the required inductance is N=40.5 turns.

From Equation 2 the power factor is P=0.23.

From Equation 5 R=6800 ohms.

The ratio of the minimum-to-maximum response over the pass band of the selector approaches unity in cases where there is a sub-' stantially uniform response. For a value of D equal to 0.945, that is, for a selector circuit having a substantially uniform response over the pass band, Equations 1 and 2, respectively. reduce to the following equations:

Assuming the above relationships, Equations 3 and 4 reduce to the following:

tions 1 and 2, respectively, reduce to the following:

Assuming the above relationships, Equations 3 and 4 reduce to the following:

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a band-pass selector for passing a wide band of frequencies, a transformer comprising two similar abutting single-layer windings of uniform pitch wound in the same direction and having the diameter of said windings, the number of turns per winding, and the pitch of the windings relatively proportioned to provide a predetermined self-inductance for each of said windings and a predetermined coefficient of coupling between said windings, said coefficient of coupling being within the limits of 0.5 and 0.1.

2. In a band-pass selector for passing a wide band of frequencies, a transformer comprising two abutting single-layer windings of uniform pitch having the same electrical and physical dimensions, the diameter of said windings, the number of turns per winding, and the pitch of the windings being relatively proportioned to provide a predetermined self-inductance for each of said windings and a predetermined coefficient of coupling between said windings, said coefficient of coupling being within the limits of 0.5 and 0.1.

3. In a band-pass selector for passing a wide band of frequencies, a transformer comprising two windings which are effectively a continuous winding of constant pitch broken at the midpoint, the diameter of said windings, the number of turns per winding, and the pitch of the windings being relatively proportioned to provide a predetermined self-inductance for each of said windings and a predetermined coeflicient of coupling between said windings, said coefficient of coupling being within the limits of 0.5 and 0.1.

i. In a band-pass selector having a substantially uniform response over a predetermined wide range of frequencies and comprising coupled tuned circuits shunted by predetermined resistances and tunedby predetermined capacitances, a transformer comprising similar primary and secondary windings, a cylindrical form for said windings, said windings being of uniform pitch and being wound in the same direction in a single-layer abutting relationship on said form, the number of turns, the pitch of said windings and the diameter of said form being relatively proportioned so that said transformer has a coefficient of coupling between the limits of 0.5 and 0.1.

5. In a band-pass selector having a predetermined ratio between maximum and minimum gain over a given wide range of frequencies of a given mean frequency and comprising coupled tuned circuits shunted by resistances of predetermined value and tuned to said mean frequency, a transformer comprising two similar abutting single-layer windings of uniform pitch wound in the same direction and having the diameter of said windings, the number of turns per winding, and the pitch of said windings relatively proportioned so that said transformer has a coefficient of coupling between said windings between the limits of 0.5 and 0.1.

6. In a band-pass selector for passing a band of frequencies having a width W and a mean frequency M with a ratio D of minimum to maximum gain over the pass band and comprising coupled tuned circuits having a coupling between said circuits substantially as given by the expression said circuits being tuned to the mean frequency of said band and being shunted by resistances to have a power factor substantially as given by the expression a transformer comprising primary and secondary windings, a cylindrical form for said windings having a diameter substantially as given by the expression.

2V22.21 Lt(11( t(1k) and each of said windings having a number of turns substantially as given by the expression Where the various symbols are as defined in the specifications, said windings having uniform pitch and being wound in a single-layer abutting relationship on said form.

'7. In a band-pass selector for passing a band of frequencies having a width W and a mean frequency M with a substantially uniform gain over the pass band and comprising coupled tuned circuits tuned to said mean frequency and shunted by resistances to have a power factor substantially as g ven by the expression a transformer comprising primary and secondary windings, a cylindrical form for said windings having a diameter in inches substantially as given by the expression zi/zazkLfli-k t(1 k) and each of said windings having a number of turns substantially as given by the expression /2kLt(1-k k said windings being of uniform pitch and being wound in a single-layer abutting relationship on said form and having a coefficient of coupling between said windings of substantially where the various symbols are as defined in the specification.

8. In a band-pass selector adapted to pass a band of frequencies having a width W and a mean frequency M with a ratio of minimum to maximum gain over the pass band greater than 0.75 and comprising coupled tuned circuits tuned. to said mean frequency and shunted by resistances to have a power factor less than 0.175, a transformer comprising primary and secondary windings having a diameter in inches equal to or greater than W 1f l.79

and each of said windings having a number of turns equal to or less than zi/easu t and each of said windings having a number of turns greater than the various symbols being as defined in the specification, said windings being of uniform pitch and being wound in a single-layer abutting relationship, said transformer having a coefficient of coupling of less than 0.5 and greater than 0.1.

MADISON CAWEIN.

v CERTIFICATE OF CORRECTION. Patent No. 2, l7l+,255. September 26, 1959.

MADISON CAWEIN- Itlis hereby certified that error appears in the printed specification I of the above numbered patent requiring correction as follows: 'Page 2, second column, line 22, Equation 1;, for N" read N; lines 14.51 and 53, in the equations, for "p read P page L first column, lineQ after the word "expression" strike out the period; and that the said Letters Patent should be read with this correction therein that the same ma; conform to the rec- 0rd of the case inthe Patent Office.

Signed and sealedthis 26th day of December,. A. D. 1959.

Henry Van Arsniale, (Seal) Acting Commissioner of Patents.

CERTIFICATE OF CORRECTION;

Patent N0. 2 1,7h,2 5. September 26;: 1959.

MADISON CAWEIN.

It .1 hereby fcr-tified that error appears in the printed specificaltion of the above numbered patent requiring" correction es follows: Page 2 second column, line 22, Equation L for N" read N; lines 15: and 55 in the eqhations, for "p 7 read P page L;, first 0011111111, 11116 6Q,ei"certhe word "expressioh" strike out theperiod; and that the said Letter s Patent should be read with this correction therein that the same may conform to the rec- 0rd of the case in the Patent Office.

Signedand sealed {this 26th day of December, A. D. 1959.

. "Henry Van Aredale, (Seal) Acting C onuniss ioner of Patents. 

